doc/go1.15: add release notes for math/cmplx

Updates #37419

Change-Id: Id7c9aba518c826c1a6fccbbf82210072bd3346f3
Reviewed-on: https://go-review.googlesource.com/c/go/+/242903
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Reviewed-by: Robert Griesemer <gri@golang.org>

Author: bmkessler

doc/go1.15.html

@@ -703,6 +703,16 @@ <h3 id="minor_library_changes">Minor changes to the library</h3>
   </dd>
 </dl><!-- math/big -->
 
+<dl id="math/cmplx"><dt><a href="/pkg/math/cmplx/">math/cmplx</a></dt>
+  <dd>
+    <p><!-- CL 220689 -->
+      The functions in this package were updated to conform to the C99 standard
+      (Annex G IEC 60559-compatible complex arithmetic) with respect to handling
+      of special arguments such as infinity, NaN and signed zero.
+    </p>
+  </dd>
+</dl><!-- math/cmplx-->
+
 <dl id="net"><dt><a href="/pkg/net/">net</a></dt>
   <dd>
     <p><!-- CL 228645 -->